Elastomers are polymeric materials characterized by their ability to quickly recover from large deformations. This elasticity is commonly increased by the vulcanization or cross-linking of the elastomer. Vulcanization is typically carried out with sulfur systems such as accelerated-sulfur vulcanization or with poly-functional resin systems such as phenol-formaldehyde-resin curatives.
For example, U.S. Pat. No. 4,774,288 employs resin systems to cure elastomers having low levels of unsaturation. As is commonly known in the art, this vulcanization is carried out in the presence of a catalyst to achieve proper cross-linking within a reasonable time. Typically, these catalysts include zinc oxide or Lewis-acid catalysts, such as stannous chloride, zinc chloride, or an equivalent thereto.
It is also common to blend elastomers with thermoplastic resins to achieve a thermoplastic elastomer. Thermoplastic elastomers are block copolymers or polymer blends characterized by their elastomeric properties, i.e. the ability to recover from deformation, as well as their thermoplastic properties, i.e., the ability to be melt-processed and reprocessed. For example, acrylonitrilebutadiene rubber (NBR) can be blended with polycaprolactam (nylon-6) to form a thermoplastic elastomer by dynamic vulcanization. Furthermore, a thermoplastic elastomer can comprise of a blend of a conventional elastomer with a thermoplastic elastomer such as an ether-ester block copolymer blended with polypropylene.
Typically, the elastomeric properties of thermoplastic elastomers are enhanced if the blend contains a vulcanizable elastomer that is cross-linked to some degree. These vulcanized thermoplastic elastomers are commonly referred to as thermoplastic vulcanizates, which are commonly produced by dynamic vulcanization. Dynamic vulcanization refers to the process whereby rubber is vulcanized during its melt-mixing with molten non-crosslinking plastic. As with the conventional vulcanization of unsaturated elastomers, the dynamic curing of unsaturated elastomers by phenolic-resin curatives requires an activation catalyst to achieve cross-linking within a reasonable time.
In many applications, however, it is undesirable to have catalysts present during the dynamic vulcanization or in the vulcanizate itself. The presence of catalysts is undesirable during dynamic vulcanization of a thermoplastic elastomer because the catalysts can catalyze undesirable reactions within the blend, particularly reactions with the non-vulcanizing thermoplastic employed. For example, certain metal halides have been known to cause transesterification when the non-vulcanizing polyesters or ester-ether block copolymers are used. Furthermore, some catalysts are corrosive and tend to degrade the thermoplastic ester polymer.
Thus, there is a need for a vulcanizable elastomeric composition which may be vulcanized in the absence of any Lewis-acid catalyst, zinc oxide, or an equivalent thereto, often referred to as activation catalysts.